Open a Gateway
Osiris believes you've impacted the timeline. Whether that's true or not, he's picked up a distress signal from Saint-14's Ghost emanating from inside the Vex Gate Network. Osiris suspects there is a gateway to the network on Nessus. Head there and find a way inside.
"We need quick access to the Vex network, and I've discovered a gateway on Nessus that is a perfect low-traffic entry point. I've armed you with some Vex transponders that will force the gateway open with a call-and-response sequence." —Osiris